Energy Efficient Lighting
Introduction
In the U.S., about 19% of all electricity is used for lighting (ACEEE, 2000). Typical
lighting power densities are about 1-2 W/ft2 in the commercial sector and 1-1.5
W/ft2 in the manufacturing sector (UD-IAC, 2008).
Modern lighting systems are much more energy efficient that previous systems.
Thus, upgrading older lighting presents an opportunity for energy savings.
However, an effective lighting system must do more than deliver light efficiently.
Effective lighting must provide the right quantity of light, with good color
rendition and minimal glare. Quality lighting and day lighting has been shown to
improve productivity and enhance worker satisfaction. In most cases, small
improvements in worker productivity greatly outweigh lighting energy costs.
Thus, when considering upgrades to reduce lighting energy use and costs, it is
essential that the changes maintain or improve the quality of light.
Inside-Out Approach to Energy Efficient Lighting
The whole-system inside-out approach to identifying savings opportunities leads
to the greatest savings at the least first cost. First, develop a baseline by
documenting the current lighting system. Next, look for savings opportunities
when current lighting is not providing useful light. Next, investigate the lighting
distribution system, including light position and fixture efficiency. Finally, consider
the actual energy conversion equipment, the lights, and look for more energy
efficient options.
To develop a lighting baseline:
 Record lighting level (fc), light quality (CRI), lighting type, input power and
occupied hours for each area within a facility. Compare measured lighting
levels with those recommended by the Illuminating Engineering Society.
 Evaluate daylighting options. Record fenestration area, orientation, and
transmittance. Record lighting levels in daylit spaces with and without
additional electric lighting. Note glare issues. Evaluate quality of light in
daylit spaces compared to other spaces.
End use lighting savings opportunities include:






Turn off blocked lights and lights in unoccupied areas.
Use motion sensors to turn off lights in seldom used areas such as
warehouses.
Turn off unnecessary lights near windows or skylights.
Use photo sensors to turn on/off outdoor lights.
Determine required light level and disconnect lights in overlit areas.
Replace colored glass and fiberglass with corrugated polycarbonate, and
turn off unnecessary lights.
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
Add windows or skylights, and turn off unnecessary lights.
Next, consider the lighting distribution system. Savings opportunities include:





Clean dirty lenses and replace yellowed lenses.
Add reflectors to fluorescent strip lights.
Add task lighting over critical areas and decrease general lighting.
Lower lights beneath scaffolding.
Paint ceilings and walls lighter color.
Finally, consider lighting replacement options. Savings opportunities include



Replace incandescent and halogen lights with compact fluorescent lights.
Replace T12 fluorescent lights with T8 or T5 lights and electronic ballasts.
Replace HID lights with HBF lights.
Productivity and Lighting
Several studies have documented productivity improvements due to lighting
upgrades and the increased use of sunlight for interior lighting. A few examples
are listed here.
Lighting Upgrades
In the post office in Reno, Nevada, a lighting retrofit with a six-year payback
increased the number of letters sorted per hour by 6% and decreased the rate of
sorting errors to 0.1% making the Reno Post Office the most efficient in the
Western US. Energy savings were about $22,400 per year, but the increase
productivity was worth about $400,000 per year (Romm and Browning, 1999).
Boeing recently went through a lighting upgrade with a two-year payback. In
addition to energy savings, “the things that people tell us are almost mind
boggling”, said one manager. Machinists report being able to read calipers and
tools more easily. The improved contrast improves workers ability to detect
imperfections in the shop by 20%. This is important because “most of the
errors… weren’t picked up until installation in the airplane, where it is much more
expensive to fix” (Romm and Browning, 1999).
Hyde Tools lighting upgrade reduced electricity costs by $48,000 per year.
However, “the quality of work improved significantly because we could see things
we couldn’t see before”. The manager estimates that the improved lighting
results in about $250,000 per year in additional revenue (Romm and Browning,
1999).
Pennsylvania Power and Light saved $2,000 per year from their lighting upgrade,
but the time required to produce drawings decreased, saving them another
$42,000 per year. In addition, sick leave decreased from 72 to 54 hours per year.
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It was apparent that “low quality seeing conditions were causing morale problems
among employees” (Romm and Browning, 1999).
West Bend Mutual Insurance Company’s move to a new building with advanced
HVAC and better lights reduced electricity costs from $2.16 /ft2 to 1.32 /ft2, and
improved productivity by 16.8% (Romm and Browning, 1999).
Day Lighting
The Herman Miller company built a new manufacturing plant with a large portion
of its lighting provided by skylights. Production output has consistently been over
20% higher than in the previous electrically-lighted plant. Employees report that
working conditions are excellent (McDonough, 2008).
Lockheed Missiles and Space Company commissioned a new office building in
California in which the cost of extensive day lighting added an extra 4% to the
buildings cost. However, electricity costs will be reduced by about $500,000 per
year for a four- year payback. In addition, absenteeism dropped by 15%, which
paid for 100% of the first cost in the first year (Romm and Browning, 1999).
Skylights were installed on one half of the roof of a Wal-mart in Lawrence, Kansas.
Sales data indicated that the departments under the skylights sold more than the
departments under the electric lighting, and more than similar departments in
other stores. To test whether this was due to the skylights, the departments were
switched from one side of the store to the other. As before, sales from
departments under the sky lights increased (Romm and Browning, 1999).
A 1999 study conducted for Pacific Gas and Electric evaluated elementary student
test scores and found that, in classrooms with daylight, test scores improved by
over 20+%. Retail sales in a chain of 100+ similar stores were also evaluated.
Sales were found to be as much as 40% higher in stores with skylighting
(Heschong Mahone Group, 1999).
Lighting Fundamentals
Effective, energy-efficient lighting systems provide the right quantity of light, with
good color rendition and minimal glare, while minimizing energy-use. Each of
these concepts is described briefly in the sections that follow.
Light Quantity
The quantity of visible light radiated by a light source is measured in lumens. The
theoretical upper limit for the conversion of energy to light is 683 lm/W. Natural
daylight has luminous efficacy of about 110 lm/W. Electric lighting ranges from
about 10 to 100 lm/W.
Illuminance is the quantity of light divided by the area on which it is incident.
Illuminance can be measured by light meters. The common measure of
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illuminance in U.S. units is footcandles. One footcandle is one lumen per square
foot.
1 fc = 1 lm/ft2
The Illuminating Engineering Society of North America (IESNA) publishes
recommended lighting levels for various tasks and spaces. IESNA recommended
lighting levels for some common spaces are shown in the table below (IESNA
Handbook, 9th Edition). In general, recommended lighting levels increase as the
size and contrast of the visual task decrease. Thus, the recommended lighting
level will be near the lower level of the range shown in the table below when the
size and/or contrast of the visual task is large, and will be near the upper level of
the range when the size and/or contrast of the visual task is small. For example,
15 fc may be sufficient for warehouses with large bulk items, but 25 fc may be
needed for warehouses with hand-stocked items. Similarly, 30 fc may be
sufficient for general manufacturing, but 50 fc may be required for manufacturing
tasks requiring visual precision.
Space/Function
Offices and classrooms
Corridors
Restrooms
Dining rooms
Merchandise Display
Warehouse
Manufacturing
Inspection
Recommended Lighting Level (fc)
30-50
5-10
5
10
50
5-30
30-50
50-100
Light Quality
Our eyes evolved to see in natural sunlight; thus, we distinguish colors best in
sunlight. Light from electric lamps is generated at lower temperatures than
sunlight and reduces our ability to distinguish between colors. Color Rendering
Index (CRI) describes the effect of a light source on the color appearance of an
object. CRI varies between 0 and 100. Approximate CRIs of various types of
lighting are shown in the table below.
Light Type
Sunlight
Incandescent
T8 Fluorescent
Metal halide
T12 Fluorescent (cool white)
High-pressure sodium
CRI
100
99
75-85
65
60
22
Some tasks, such as inspection and painting, clearly require high-quality light. In
addition, most people prefer to work and live in light that is as close to sunlight as
Energy Efficient Lighting
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possible; thus the CRI of a light source should always be a consideration when
selecting lights. For example, most people report seeing better under fluorescent
lights with a CRI of 85 than under high-pressure sodium lights with a CRI of 22,
even when the illuminance under the high-pressure sodium lights is higher.
Glare
Glare is uncomfortably high illuminance. Glare can be problematic with large
windows with direct sunlight and with direct high-intensity artificial lighting. For
this reason, windows are often equipped with some type of shading and
luminaries are often designed to diffuse light.
Electric Lighting
Common types of electrical lighting fall into three categories: incandescent,
fluorescent and high-intensity discharge. Important characteristics of each
category are described below.
Incandescent Lights
Thomas Edison invented the incandescent light bulb and it remains essentially
unchanged today. Incandescent lights produce light by simple radiation from a
heated tungsten filament. The lighting efficiency is generally low since most of
the energy is released as infrared radiation rather than visible light. In recent
years, halogen has been added to the filament increasing the efficiency and light
output by about 30% in ‘halogen lights’. Incandescent lights are inexpensive,
require almost no warm-up time, and the lighting output does not degrade
significantly over time. However, because of their inefficiency, they are being
replaced by compact fluorescent lights that use about 30% as much energy and
last about seven times longer.
Fluorescent Lights
Fluorescent lights work by energizing Ar, Ar-Ne or Kr gasses inside a tube. The
gasses produce UV radiation that is converted to visible light when it interacts
with phosphor coatings on the inside of the tube. Fluorescent lights have a higher
lighting efficiency than incandescent lights. They start quickly and lighting output
degrades only moderately over time.
Fluorescent lamps are labeled using “F” and “T” notion. The “F” notation refers to
the nominal wattage of the lamp. The “T” notation denotes the diameter in units
of 1/8 inch. Thus, Thus ‘F32T8’ refers to a lamp with nominal power draw of 32 W
and 1 inch diameter. Over time, the progression has been toward thinner lamps,
from T12 to T8 to T5, with corresponding improvements in energy efficiency and
color rendition. Tubular fluorescent lamps with low mercury content are generally
marked with green end caps.
All fluorescent lights require ballasts. The ballast regulates voltage and uses some
energy itself. Old-style T12 lamps used magnetic and electro-magnetic ballasts. T8
and T5 lamps use electronic ballasts. New electronic ballasts are more energy-
Energy Efficient Lighting
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efficient, eliminate the flicker associated with old-style magnetic ballasts, are
quieter and contain less heavy metals than old-style ballasts.
The actual power draw of the lamp, and the corresponding lamp output are
determined by the power supplied to the lamp by the ballasts. The relative power
delivered by the ballast to the lamps is called “ballast factor”. For example, lamps
powered by a ballast with a ballast factor of 0.87, will draw about 87% of their
rated wattage and deliver about 87% of their rated lighting output.
The fixture that houses the lamps and ballast and directs the light onto the work
plane is called a luminaire. The most common types of fluorescent luminaries are
recessed-troffer and suspended luminaires. Many offices employ recessed-troffer
luminaires. The troffers are typically equipped with acrylic lenses or parabolic
grids to diffuse the light. Although acrylic lenses transmit more light than
parabolic grids, parabolic grids produce less glare and are therefore widely used in
rooms with video displays.
Recessed parabolic and lensed troffers.
In industrial applications, fluorescent luminaires are typically suspended from the
ceiling. Simple “strip” lights without reflectors attached to the sides lose a
significant amount of the light sideways and upwards. Adding reflectors to the
sides of the luminaire pushes more light downward onto the work plane, and
reduces the number of lights required to generate a given lighting level on the
work plane. The most efficient fluorescent luminaires employ polished metal
mirrors above each lamp to direct the maximum possible light onto the work
plane. High-bay fluorescent (HBF) luminaires employ these polished reflectors,
and are displacing HID lights in industrial applications due to their improved
energy efficiency, color rendition and other attributes.
A) fluorescent strip light, B) fluorescent strip light with side reflectors, C) high-bay
fluorescent light with individual polished mirror reflectors.
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High-Intensity Discharge Lights
High-intensity discharge (HID) lights are primarily used in applications with high
ceilings or for outdoor lighting. The most common types of HID lights are metal
halide (MH) and high-pressure sodium (HPS). MH lights produce a white light with
a CRI of about 65, and have a lighting efficiency of about 70 lm/W. HPS lights have
a lighting efficiency of about 95 lm/W, but produce a yellow light with poor color
rendering (CRI = 20); thus, these lights are best suited for outdoor and warehouse
applications.
All HID lamps require a ballast. The ballast for a typical 400-W HID lamp uses 50
to 65 W. HID lamps take about 3 minutes to start-up when cold and about 25
minutes when warm. Thus, most HID lighting systems include a few fluorescent
safety lights that start up immediately in the advent of a power interruption.
Two-stage and dimmable HID ballasts with occupancy sensors are available, Twostage HID ballasts typically have a low-lighting stage that produces 35% of the
light while using 50% of the rated energy.
Luminaries for HID lights typically use acrylic or spun-aluminum reflectors. Acrylic
reflectors spread about 10% of the light horizontally and toward the ceiling.
Because of this, they are often used in retail applications where bright ceilings are
more visually attractive. Spun aluminum reflectors are the most common type in
industrial applications and direct all light toward the floor. HID luminaries are
classified as high-bay for placements higher than about 25 feet, and low-bay for
placements less than about 25 feet. Low-bay luminaries have acrylic lenses that
spread the light outward over a wider surface area.
A)
acrylic high-bay luminaire, B) aluminum high-bay luminaire, C) aluminum
low-bay luminaire with acrylic lens.
Comparative Characteristics
The efficiency of converting electricity to light can be measured as the ratio of
light output (lm) and electrical power (W). All common types of electric lights
except incandescent lights require a ballast to regulate the voltage to the lamp.
Thus, the lighting efficiency should include the electricity consumed by both the
lamp and ballast. The lighting output of most types of lights degrades over the
lifetime of the light, thus the mean lighting output should be used when
calculating energy efficiency. The approximate lighting efficiencies of common
Energy Efficient Lighting
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types of lighting systems (including ballasts), and other characteristics are shown
in the table below.
Light Type
Efficiency
(Lm/W)*
18
70
58
100
70
96
CRI
Lifetime
(hours)
750
10,000
20,000
20,000
20,000
20,000
Lumen
Maint.
Good
OK
Good
Good
Fair
Good
Restrike
Time
Instant
Immediate
Immediate
Immediate
30 min
30 min
Incandescent
99
Compact fluorescent
60
T12 Fluorescent
60
T8 Fluorescent
75
Metal halide
65
High-pressure
22
sodium
*Typical lm/W calculated using mean lighting output (lm) and energy use (W)
including ballast.
Performance Metrics
Typical performance and cost data for common lamps and ballasts are shown
below. Fluorescent and HID lamps are generally specified by a nominal power
draw and nominal light output. However, the power to a lamp is regulated by a
ballast. Thus, the actual power draw of the lamps plus the ballast, and light
output of the lamps is determined by the ballast.
Example
4-ft T8 fluorescent lamps have a nominal power draw of 32 W each
and nominal light output of 2,710 lm each. Using data from the tables
shown below, calculate the actual power draw and lighting output of
two 32-W lamps when powered by a single low-output electronic
ballast, and when powered by a high-output electronic ballast.
The power consumption of a single low-output electronic ballast is 51
W and the power consumption of a single high-output electronic
ballast is 77 W. Thus,
Power input of 2 lamps and the ballast for low output ballast = 51 W
Power input of 2 lamps and the ballast for high output ballast = 77 W
The actual light output of a lamp is the product of the nominal rated
light output and the ballast factor associated with the ballast. Thus,
the actual light output of two 32-W lamps with nominal rated output
of 2,710 lm each, when powered by a low-output electronic ballast
with ballast factor 0.75 is:
Light output (low output ballast) = 2 lamps x 2,710 lm/lamp x 0.75 =
4,065 lm
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If the same lamps were powered by a high-output electronic ballast
with ballast factor 1.20 the actual light output would be:
Light output (high output ballast) = 2 lamps x 2,710 lm/lamp x 1.20 =
6,504 lm
Similarly, a metal halide lamp may be labeled with a nominal power draw of 400
W and nominal light output of 23,500 lm. However, as shown in the tables below,
the lamp and the ballast together draws 460 W. HID ballasts generally have a
ballast factor of 1.0. Thus, the actual light output of HID lamp is equivalent to the
nominal light output of the lamps.
Fluorescent Lamps
Type
4-ft T12
48-in T12 34-W
48-in T12 40-W
4-ft T8
48-in T8 32-W
48-in T8 32-W, long life, low merc
48-in T8 32-W, cover guard
8-ft T12
96-in T12 60-W
96-in T12 95-W
96-in T12 110-W
8-ft T8
96-in T8 59-W
96-in T8 59-W, cover guard
96-in T8 86-W
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Nominal
Power
(W)
Rated
Life
(hr)
Mean
Output
(lm)
CRI
Cost
($)
34
40
20,000
20,000
2,280
2,910
62
73
1.40
4.00
32
32
32
20,000
24,000
20,000
2,710
2,710
2,625
78
75
78
1.90
2.60
11.00
60
95
110
12,000
12,000
12,000
5,060
6,960
7,740
62
60
60
3.90
6.00
13.00
59
59
86
15,000
15,000
18,000
5,310
5,150
7,200
75
75
75
7.80
24.10
17.70
Fluorescent Ballasts
Type
Lamps
Lamp
Power
(W)
Ballast
Power
(W)
Ballast
Factor
2
34
68
.87
2
2
2
32
32
32
51
58
77
.75
.87
1.20
2
2
2
60
95
110
112
203
237
.88
.91
.95
2
2
59
86
110
160
.85
.88
4-ft T12
Fluor F34T12 Electromagnetic
4-ft T8
Fluor F32T8 Electronic (Low Output)
Fluor F32T8 Electronic (Normal Output)
Fluor F32T8 Electronic (High Output)
8-ft T12
Fluor F96T12 Electromagnetic
Fluor F96T12 Electromagnetic
Fluor F96T12 Electromagnetic
8-ft T8
Fluor F96T8 Electronic
Fluor F96T8 Electronic
Cost
($)
$36
$15
$36
$25
$29
Metal Halide Lights
175 W
250 W
360 W
400 W
1,000 W
1
1
1
1
1
kW/fix
0.210
0.295
0.420
0.460
1.080
Lum/fix
8,800
13,500
23,500
23,500
66,000
Hours/lamp
10,000
10,000
20,000
20,000
12,000
Cost/lamp
$27
$26
$50
$23
$74
Cost/fix*: Low Bay
$225
$215
$250
$250
$205
Cost/fix*: High Bay
--65
--65
$110
65
$110
65
$235
65
Lamps/fix
CRI
High-Pressure Sodium Lights
70 W
100 W
150 W
250 W
400 W
1,000 W
1
1
1
1
1
1
kW/fix
0.088
0.128
0.185
0.300
0.465
1.110
Lum/fix
5,450
8,550
14,400
27,000
45,000
126,000
Hours/lamp
24,000
24,000
24,000
24,000
24,000
24,000
Cost/lamp
$27
$23
$21
$21
$19
$83
Cost/fix*
$163
22
$165
22
$160
22
$196
22
$216
22
$360
22
Lamps/fix
CRI
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High-Bay Fluorescent Lights
T8
T8
T8
T5
T5
T5
4
6
8
4
6
8
kW/fix
0.156
0.234
0.312
0.236
0.354
0.472
Lum/fix **
13,680
20,520
27,360
24,000
36,000
48,000
Hours/lamp***
24,000
24,000
24,000
24,000
24,000
24,000
Cost/lamp
$2.50
$2.50
$2.50
$15
$15
$15
Cost/fix*
$120
$150
$200
$170
$230
$300
Cost/occ sen
$70
$70
$70
$70
$70
$70
CRI
85
85
85
85
85
* Fixture cost includes lamp, ballast and luminaire
** Assumes three 2-lamp, 1.2 BF, 78-W ballasts
***Assumes long-life lamps rated at 24,000 hours with cycle of 3 hours/start.
85
Lamps/fix
Lighting Design
The Illuminating Engineering Society (IES) lumen method calculates the
illuminance on a workplane, Ew (fc), as:
Ew = Cu x F / Aw
where Cu is the coefficient of utilization, F is the total lumens produced by the
lamps, and Aw is the area of the work plane. Cu is a calculated from a
manufacturer-supplied table based on the type of luminaire, room geometries
and surface reflectivities. Typical Cus range from about 0.2 to 0.6 (Kreider and
Rabl 1994) Heating and Cooling of Buildings.
This equation can also be written as
Ew = Cu x (LPF x N) / Aw
Where LPF is the lumens per fixture and N is the number of fixtures. In
fluorescent lights, lumens per fixture LPF is the product of the number of lamps,
NL, lumens per lamp, LPL, and the ballast factor, BF.
LPF = NL x LPL x BF
To determine how many fixtures are needed simply solve this equation for N.
N = (Ew x Aw) / (Cu x LPF)
Values of the coefficient of utilization, CU, for typical 4-lamp fluorescent troffer
fixtures, 400-W high-bay metal halide fixtures and 6-lamp high bay fluorescent
fixtures are shown in the tables below.
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CU values for recessed 2’ x 4’ fixture with acrylic lens and 4 lamps
(www.columbialighting.com)
CU values for 8-ft 4-lamp or 4-ft 2-lamp fluorescent fixture (www.goodmart.com)
CU values for high-bay 400-W MH or HPS fixture (www.cooperlighting.com)
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CU values for high-bay six-lamp ORION 416 HPM fluorescent fixture
(www.orionlighting.com)
In each table CU depends on the room cavity ratio, RCR, where
RCR = 5 x h x (w + l) / (w x l)
CU also depends on the reflectivity of the ceiling, rc, and the reflectivity of the
walls, rw. Most paint color chips report Light Reflecting Value (LRV), which is the
reflectivity. Light surfaces have a high reflectivity.
Source: http://www.squidoo.com/LRV
Electrical Lighting Design Example
Determine the number of 400-W metal halide fixtures and 230-W high-bay
fluorescent fixtures required to light a space with the following
characteristics:
Ew = 40 fc
W = width = 50 ft
L = length = 100 ft
H = height = 25 ft
Rc = reflectivity of ceiling = 50%
Rw = reflectivity of walls = 50%
Solution:
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RCR = 5 x h x (w + l) / (w x l) = 5 x 25 x (50 + 100) / (50 x 100) = 3.75 ~ 4.0
CU (400-W metal halide) = 0.57
CU (230-W high-bay fluorescent) = 0.69
LPF (400-W metal halide) = 23,500 lm
LPF (230-W high-bay fluor, w/ 3 2-lamp, 1.2 BF, 78-W ballasts at 8,000 hrs) =
3 x 6,840 lm = 20,520 lm
N = (Ew x Aw) / (Cu x LPF)
N (400-W metal halide) = (40 x [50 x 100]) / (.57 x 23,500) = 14.9 ~ 15 lights
N (230-W high-bay fluorescent) = (40 x [50 x 100]) / (.69 x 20,520) = 14.1 ~
14 lights
Lamp Replacement Costs
Overtime, lamps fail and need to be replaced. Thus, economic analysis of lighting
upgrades should consider lamp replacement costs.
The number of lamps that must be replaced each year, Nr, can be calculated as:
Nr = Nt x AOH / LL
Where:
Nt = total number of lamps in operation
AOH = annual operating hours
LL = lamp lifetime
Lamp Replacement Cost Example
Consider lamp replacement savings from replacing 320 400-W MH fixtures
with 320 6-lamp HBF fixtures if the lights operate 8,000 hours per year.
The number of lamp replacements would be about:
MH: (320 fix x 1 lamps/fix) x 8,000 hours/year / 20,000 hours = 128
lamps/year
T8: (320 fix x 6 lamps/fix) x 8,000 hours/year / 30,000 hours = 512
lamps/year
The cost of a 400-W MH lamp is about $23, and T8 lamps cost about $3
each. The hourly wage for a skilled-trade electrician, including all benefits,
is about $65 per hour. Assuming it would take one worker about 30
minutes to replace either a single MH lamp or 6 T8 lamps, the annual
maintenance costs are about:
MH: 128 lamps/year x ($23 /lamp + (30/60 hours/lamp x $65 /hour)) =
$7,104 /year
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T8: 512 lamps/year x ($3 /lamp + (30/60 hours / 6 lamps x $65 /hour)) =
$4,309 /year
Thus, the annual maintenance savings would be about:
$7,104 /year - $4,309 /year = $2,795 /year
Economics of Lighting Upgrades
The methods described above can be used to determine the economics of lighting
upgrades. The method typically involves the following steps.






Calculate how many of the proposed lights are needed to deliver the
required number of footcandles in the space.
Calculate the annual energy cost savings of replacing the current lights
with the proposed lights.
Calculate the annual relamping cost savings, including both labor and
material costs.
Calculate total annual cost savings including both annual energy and
annual relamping savings.
Calculate the one-time implementation cost of replacing the current lights
with the proposed lights.
Calculate the simple payback of the investment.
The performance and coefficient of utilization data from the preceding sections
can be used in these calculations.
Lighting Upgrade Economics Example
A facility has 25 fixtures with 400-W high pressure sodium (HPS) lamps. A
lighting upgrade to 6-lamp T8 high-bay fluorescent (HBF) fixtures is proposed.
The lights operate 6,000 hours per year and the average cost of electricity is
$0.10 /kWh. The hourly wage for an electrician is $50 per hour. It takes an
electrician 15 minutes to replace either a single HPS lamp or 6 T8 lamps. It
takes an electrician 30 minutes to replace a HPS fixture with a new HBF
fixture.
Determine annual cost savings including energy and lamp replacement costs,
the implementation cost, and simple payback of the project. The space has
the following characteristics:
Ew, required = 50 fc
W = width = 150 ft
L = length = 100 ft
H = height = 25 ft
Rc = reflectivity of ceiling = 70%
Rw = reflectivity of walls = 70%
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Solution:
The number of HBF fixtures required would be:
RCR = 5 x h x (w + l) / (w x l) = 5 x 25 x (150 + 100) / (150 x 100) = 2.0
CU (HBF) = 0.93
LPF (HBF) = 20,520 lm
N (HBF) = (Ew x Aw) / (Cu x LPF)
N (HBF) = (50 x [150 x 100]) / (.93 x 20,520) = 34 fixtures
The energy cost savings would be:
Esav = [(25 fix x .465 kW/fix) – (34 fix x .234 kW/fix)] x 6,000 hr/yr
Esav = 22,014 kWh/yr
ECsav = 22,014 kWh/yr x $0.10 /kWh = $2,201 /yr
The number of annual lamp replacements would be:
HPS: (25 fix x 1 lamps/fix) x 6,000 hours/year / 24,000 hours = 6 lamps/year
HBF: (34 fix x 6 lamps/fix) x 6,000 hours/year / 24,000 hours = 51 lamps/year
The annual lamp replacement costs would be:
HPS: 6 lamps/year x ($19 /lamp + (.25 hr/lamp x $50 /hr)) = $189 /yr
HBF: 51 lamps/year x ($2.5 /lamp + (.25 hr/ 6 lamps x $50 /hr)) = $234 /yr
The annual lamp replacement savings would be:
$189 /year - $234 /year = -$45 /year
The total annual savings would be:
$2,201 /year - $45 /year = $2,157 /year
The implementation cost and simple payback would be:
IC = 34 fix x [$200 /fix + (.50 hr/fix x $50 /hr)] = $7,650
SP = $7,650 / $2,157 /yr x 12 mo/yr = 43 months
Natural Lighting Design
Natural lighting uses light through skylights in the roof of a building or windows in
the walls of a building.
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Illuminance from Skylights
The Illuminating Engineering Society (IES) lumen method to calculate the
illuminance on a work plane, Ew (fc), from the illuminance on a horizontal skylight
Esl (fc) is:
Ew = Esl x Cu x transmissivity of the skylight x transmissivity of the well x Asl / Aw
Where Cu is the coefficient of utilization, Asl is the area of the skylight and Aw is
the area of the work plane.
Cu is a function of ceiling reflectivity, pc , wall reflectivity, pw, and the room cavity
ratio, RCR. Values of Cu for skylights are tabulated in Table 13.5 of Kreider and
Rabl, 1994, Heating and Cooling of Buildings. An adequate approximation is given
by:
Cu = 1.016898 + 0.074074 pc + -0.14778 RCR + 0.111111 pw + 0.037037 pc2 +
0.008133 RCR2 + 0.125 pw2
The room cavity ratio, RCR, is:
RCR = 5 x h x (w + l) / (w x l)
where h is the height of the skylight over the work plane, w is the width of the
room and l is the length of the room.
Design values of the illuminance on a skylight, Esl, can be found in Figures 13.4 –
13.6 of Kreider and Rabl, 1994, Heating and Cooling of Buildings. Alternately, Esl
can be calculated from the total radiation on a horizontal surface Ih (Btu/hr-ft2)
by assuming that the illuminance of sunlight is 110 lm/W.
Esl (fc) = Ih Btu/hr-ft2 / 3.413 Btu/W-hr x 110 lm/W x 1 fc/(lm/ft2)
According to a local roofing company contacted in 2002, the material and
installation for a 4 ft by 4 ft skylight with a 10-foot shaft would cost about $2,000.
Illuminance from Skylights Example
A 50 ft by 50 ft room with a 25 foot ceiling has 50 ft2 of skylights.
The total solar radiation on the skylights is 300 Btu/hr-ft2. Calculate
the illuminance on the floor. The reflectivity of the walls is 0.70 and
the reflectivity of the ceiling is 0.70. The average transmissivity of
each skylight is 0.80 and the average transmissivity of each skylight
well is 0.90.
Assuming that the illuminance of sunlight is 110 lm/W, the
illuminance on a skylight, Esl, is:
Esl = Ih Btu/hr-ft2 / 3.413 Btu/W-hr x 110 lm/W x 1 fc/(lm/ft2) =
9,669 fc
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The room cavity ratio, RCR, is:
RCR = 5 x h x (w + l) / (w x l) = 5
The coefficient of utilization, Cu, is:
Cu = 1.016898 + 0.074074 pc + -0.14778 rcr + 0.111111 pw +
0.037037 pc2 + 0.008133 rcr2 + 0.125 pw2 = 0.690
The illuminance on a work plane, Ew (fc), is:
Ew = Esl x Cu x transmissivity of skylight x transmissivity of well x Asl
/ Aw = 96 fc
Illuminance from Vertical Windows
The IES recommends an illuminance of about 20 fcs for warehouse spaces and 50
fc for precision work. The general equation to calculate the illuminance on a work
plane, Ew (fc), from the illuminance on a vertical window Ev (fc) is:
Ew = Ev x Cu x transmissivity of the window
Where Cu is the coefficient of utilization. Cu is a function of the window length
and height, room depth and the distance between the window and the work
plane (expressed as a fraction of the room depth). Values of Cu for windows are
tabulated in Table 13.8 or Kreider and Rabl, 1994, Heating and Cooling of
Buildings.
Design value of the illuminance on a vertical window, Ev, can be found in Figures
13.4 – 13.6 of Kreider and Rabl, 1994, Heating and Cooling of Buildings.
Alternately, Ev can be calculated from the total radiation on a vertical surface Iv
(Btu/hr-ft2) by assuming that the illuminance of sunlight is 110 lm/W.
Ev (fc) = Iv Btu/hr-ft2 / 3.413 Btu/W-hr x 110 lm/W x 1 fc/(lm/ft2)
If the window is shaded from direct solar radiation, then the Iv should be the
diffuse component of solar radiation.
LightSim Daylighting Analysis Software
LightSim daylighting analysis software simulates hour-by-hour illuminance on a
work plane using TMY2 meteorological data. It is specifically designed to assess
the feasibility of daylighting in buildings. LightSim can quickly determine the
fraction of time that various daylighting designs can meet or exceed a target
illumination on a work plane. LightSim is available at no cost from the University
of Dayton IAC.
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Photo Controls
Photo controls turn lights on and off depending on lighting levels or occupancy. In
indoor lighting applications, photo controls are most often used with fluorescent
or incandescent lights since these lights turn on and off quickly. Photo controls
are seldom used with HID lighting in indoor applications, because of the 15minute restrike time required for HID lighting to regain full output.
Photo Sensors
Photo sensors measure light levels and control lights based on the difference
between the measured light level and a set point.
In outdoor applications, photo sensors turn lights off during the day and on during
night, with an average on-time of 12 hours per day. Most outdoor HID lights
come with a ½-inch knockout or an 11/16-inch hole for easy screw-in mounting.
Most photo sensors have built-in delays to prevent false cycling from headlights,
etc. Photo sensors typically cost less than $15 each and can be installed in less
than ½ hour. Thus, it is virtually always cost-effective to install photo sensors on
outdoor lights to prevent lights from running during daylight hours. In indoor
applications, photo sensors turn off unnecessary lights in day lit spaces.
Occupancy Sensors
Occupancy sensors turn lights on when a space is occupied and turn lights off
about 4 minutes after the space is unoccupied. Most occupancy sensors use
infrared sensors to detect body heat in motion.
In commercial buildings, occupancy sensors are especially effective in lightly used
areas, such as lavatories or conference rooms, or in rooms where lights may be
left on inadvertently. In manufacturing facilities, occupancy sensors are especially
effective in ware house areas with limited traffic between rows of stacks.
Residential occupancy sensors cost about $15 each and commercial/industrial
quality occupancy sensors cost between $80 and $200 each.
Ballast and Lamp Disposal
Ballasts manufactured before 1979 contained wet-capacitors with the hazardous
waste PCB. After 1979, ballasts used dry capacitors and contain no appreciable
hazardous wastes. Because very few pre-1979 ballasts remain, most states allow
post-1979 ballasts to be disposed of in normal waste streams.
Fluorescent and HID lamps contain small amounts of mercury in the phosphor
powder. Mercury is a potent neurotoxin and is harmful to both animals and
humans. When unbroken lamps are disposed of in landfills, virtually no mercury
leaches into the environment. However, if lamps are incinerated, the mercury is
transported through the atmosphere to water, animals and humans. Spent lamps
can also be recycled; however, some studies indicate that more mercury is
released into the environment during recycling than by placing the lamps in
landfills.
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Currently, lamp disposal is regulated on a state-by-state basis, or indirectly by the
1990 RCRA, which recognizes mercury as a hazardous waste. In many states, all
traditional spent lamps from businesses are considered hazardous waste
irrespective of the number of lamps being disposed. In this case, spent lamps
should be sent to a registered recycler or hazardous waste contractor. Under
RCRA, small quantity generators that dispose less than 350 4-foot lamps per
month can legally dispose of spent lamps into the municipal solid waste stream.
In any case, if the municipal waste is incinerated, it is recommended that the
lamps be sent to an EPA-registered recycler or hazardous waste disposal
company. Be sure to research the credibility of the recycler or waste
management company. If the waste is not disposed of properly, the original
generator may be legally liable. Recyclers and hazardous waste handlers typically
charge about 40 cents per lamp for disposal or recycling.
For new lamp purchases, specify low-mercury lamps, which are commonly marked
with green end-caps. Low-mercury lamps have about 5 milligrams of mercury
compared to about 48 milligrams for 1985 vintage lamps. The additional cost for
low-mercury lamps is usually negligible, and in most states, spent low-mercury
lamps can be deposited directly in landfills instead of being sent to a lamp
recycler. This saves disposal costs and reduces potential environmental legal
liability issues.
Compact fluorescent lamps contain about 4 mg of mercury. However, the 75%
reduction in energy use compared to incandescent lamps displaces mercury
emissions from coal fired power plants. The U.S. EPA estimates that the use of
CFLs in place of incandescent lamps results in over three times less mercury
emissions.
Source: U.S. E.P.A., Information on CFLs and Mercury, 10/2010
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Heating and Cooling Interactions
Reducing the lighting energy added to a conditioned space decreases the space
cooling load and increases the space heating load. From simple energy balances,
the air conditioning electricity savings can be estimated as:
[(Plights,pre - Plights,post) x hours/year ] / cooling
The additional heating energy can be estimated as:
[(Plights,pre - Plights,post) x hours/year] / heating
Where Plights is the power draw of the lights and  is the efficiency of the space
conditioning equipment.
Heating and Cooling Energy Example
If the lighting load were decreased from 50 kW to 40 kW, for 2,500
hours per year during the cooling season, and the average efficiency
(coefficient of performance) of an air conditioner is 3.0, calculate
the air conditioning savings:
The air conditioning savings would be:
[(50 kW - 40 kW) x 2,500 hr/yr ] / 3.0 = 8,333 kWh/yr
If the lighting load were decreased by the same amount for 2,500
hours during the heating season, and the average efficiency of a
furnace was 75%, calculate the additional heating energy needed.
The additional heating energy would be about:
[(50 kW - 40 kW) x 2,500 hr/yr ] / 0.80 x 3,413 Btu/kWh = 117 x 106
Btu/yr
Note that when the heating and cooling periods are about the same length, as in
this example, the increased air conditioning and decreased heating costs may be
nearly equal. For this reason, they are sometimes ignored. In buildings with
complex HVAC systems, heating and cooling interaction effects can be more
accurately modeled by building energy simulation software.
Emerging Lighting Technologies
Light emitting diodes (LEDs) are semiconductor materials that emit light when
electricity is passed through them. Currently, LEDs are used in thin-screen
computer monitors, thin-screen televisions, watches, exit signs, flashlights, traffic
lights and many other applications. Their appeal lies in their energy efficiency and
longevity. Current white light LEDs have efficiencies somewhere between
incandescent lights, at about 5%, and fluorescent lights, at about 25%. Current
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LEDs last about 5 times as long as incandescent lights. For example, California has
replaced thousands of 150-W incandescent light bulbs that last about 1 year in
traffic lights with red, yellow and green LEDs that consume about 15 W and last
about 5 years.
A principle challenge for white light LEDs is converting the colored light produced
by an LED into white light, which a combination of many colors of light. One
current design uses a LED that produces UV light to stimulate phosphors which
emit red, green and blue light in the proper proportion to make white light.
Another approach is to develop organic LEDs, which resemble a patch of softly
glowing plastic. Organic LEDs could someday be mass-produced using ink jet
technology on a substrate. Theoretically, LEDs could be 100% efficient.
(Technology Review, 2003).
References
American Council for an Energy-Efficient Economy, 2000, “Guide to EnergyEfficient Commercial Equipment”, Washington, D.C.
Energy Information Administration, 1996, “Residential Lighting Use and Potential
Savings”
DOE/EIA-0555(96)/2, U.S. Department of Energy, Washington, DC,
http://www.eia.doe.gov/emeu/lighting/.
Energy Information Administration, 2002, “U.S. Lighting Market Characterization,
Volume 1: National Lighting Inventory and Energy Consumption Estimate”, U.S.
Department of Energy, Washington, D.C.
Granger, 2001, “Grainger Industrial Supply Catalog 2000-2001”, No. 391.
Heschong Mahone Group, 1999, “Daylighting in Schools” and “Skylighting and
Retail Sales”, Pacific Gas and Electric, http://www.h-mg.com/Daylighting/daylighting_and_productivity.htm
Kreider, J.F. and Rabl, A., “Heating and Cooling of Buildings”, 1994, McGraw-Hill
Inc.
McDonough, 2008, http://www.mcdonough.com/miller.html.
Orion, Plymouth WI, www.orionlighting.com
Romm and Browning, 1999, “Greening the Building and the Bottom Line”, Global
Energy Conference, Vancouver, May.
Suozzo, M., Benya, J., Hydeman, M., Dupont, P., Nadel, S. and Elliot, N., “Guide to
Energy Efficient Commercial Equipment”, 2000, American Council for an Energy
Efficient Economy.
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Technology Review, 2003, “LED vs. the Lightbulb”, May, pgs. 30-36.
University of Dayton Industrial Assessment Center, 2008, Dayton, OH.
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